Optical reduction system with elimination of reticle diffraction induced bias

ABSTRACT

An optical reduction system for use in the photolithographic manufacture of semiconductor devices having one or more quarter-wave plates operating near the long conjugate end. A quarter-wave plate after the reticle provides linearly polarized light at or near the beamsplitter. A quarter-wave plate before the reticle provides circularly polarized or generally unpolarized light at or near the reticle. Additional quarter-wave plates are used to further reduce transmission loss and asymmetries from feature orientation. The optical reduction system provides a relatively high numerical aperture of 0.7 capable of patterning features smaller than 0.25 microns over a 26 mm×5 mm field. The optical reduction system is thereby well adapted to a step and scan microlithographic exposure tool as used in semiconductor manufacturing. Several other embodiments combine elements of different refracting power to widen the spectral bandwidth which can be achieved.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Pat. appl. Ser. No.10/366,614, filed Feb. 14, 2003, which is a continuation of U.S. Pat.Appl. Ser. No. 09/841,166, filed Apr. 25, 2001, which claims the benefitof U.S. Provisional Pat. Appl. No. 60/199,392, filed Apr. 25, 2000, eachof which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical systems used insemiconductor manufacturing.

2. Background Art

Semiconductor devices are typically manufactured using variousphotolithographic techniques. The circuitry used in a semiconductor chipis projected from a reticle onto a wafer. This projection is oftenaccomplished with the use of optical systems. The design of theseoptical systems is often complex, and it is difficult to obtain thedesired resolution necessary for reproducing the ever-decreasing size ofcomponents being placed on a semiconductor chip. Therefore, there hasbeen much effort expended to develop an optical reduction system capableof reproducing very fine component features, less than 0.25 microns. Theneed to develop an optical system capable of reproducing very finecomponent features requires the improvement of system performance.

A conventional optical system is disclosed in U.S. Pat. No. 5,537,260entitled “Catadioptric Optical Reduction System with High NumericalAperture” issued Jul. 16, 1996 to Williamson, which is incorporated byreference herein in its entirety. This reference describes an opticalreduction system having a numerical aperture of 0.35. Another opticalsystem is described in U.S. Pat. No. 4,953,960 entitled “OpticalReduction System” issuing Sep. 4, 1990 to Williamson, which isincorporated by reference herein in its entirety. This referencedescribes an optical system operating in the range of 248 nanometers andhaving a numerical aperture of 0.45.

BRIEF SUMMARY OF THE INVENTION

While these optical systems perform adequately for their intendedpurpose, there is an ever increasing need to improve system performance.The present inventor has identified that a need exists for eliminatingdiffraction induced by bias at the reticle. Further, there is a need foran optical system having low reticle diffraction capable of acceptablesystem performance over a large spectral waveband.

Reticle diffraction induced bias results from the way linearly polarizedlight interacts with the features of the reticle. The featureorientation of the reticle is determined by the semiconductor devicebeing projected. Since there is an increasing need to reduce the size ofsemiconductor devices and feature orientation is dictated by theapplication of the semiconductor device, the present inventor focused ontreating reticle diffraction.

Linearly polarized light is typically used in certain photolithographicprojection optic systems. Diffraction results from the interaction oflight and the features on the reticle. Linearly polarized light travelsthrough the reticle differently depending on the orientation of itsfeatures. Asymmetries result from this interaction. The asymmetries orprint biases are then projected through the optical system onto thewafer. Print bias is significant enough to alter the thickness of thelines projected on the wafer. Variations on the wafer affect theperformance of the semiconductor device, and in some cases prevent thedevice from performing to required specifications.

The use of circularly polarized light at the reticle can eliminate theasymmetries which result from feature orientation. This circularlypolarized light is indistinguishable from unpolarized light in itsimaging behavior. The imaging behavior of unpolarized light is such thatit diffracts equally regardless of the orientation of the featurethrough which it is projected. Thus the print biases are reducedthroughout the optical system.

However, other factors, such as transmission loss, prevent the use ofcircularly polarized light throughout an optical system. Thus, thepresent invention involves the use of phase shifters, which can take theform of wave plates, retardation plates and the like, to selectivelyalter the polarization of the light before the reticle and opticalsystem.

In one embodiment, the present invention is a catadioptric opticalreduction system for use in the photolithographic manufacture ofsemiconductor devices having one or more quarter-wave plates operatingnear the long conjugate end. A quarter-wave plate after the reticleprovides linearly polarized light at or near the beamsplitter. Aquarter-wave plate before the reticle provides circularly polarized orgenerally unpolarized light at or near the reticle. Additionalquarter-wave plates are used to further reduce transmission loss andasymmetries from feature orientation. The catadioptric optical reductionsystem provides a relatively high numerical aperture of 0.7 capable ofpatterning features smaller than 0.25 microns over a 26 mm×5 mm field.The optical reduction system is thereby well adapted to a step and scanmicrolithographic exposure tool as used in semiconductor manufacturing.Several other embodiments combine elements of different refracting powerto widen the spectral bandwidth which can be achieved.

In another embodiment, the present invention is a catadioptric reductionsystem having, from the object or long conjugate end to the reducedimage or short conjugate end, an first quarter-wave plate, a reticle, asecond quarter-wave plate, a first lens group, a second lens group, abeamsplitter cube, a concentric concave mirror, and a third lens group.The first quarter-wave plate operates to circularly polarize theradiation passed to the reticle. The second quarter-wave plate operatesto linearly polarize the radiation after the reticle before the firstlens group. The concave mirror operates near unit magnification. Thisreduces the aberrations introduced by the mirror and the diameter ofradiation entering the beamsplitter cube. The first and second lensgroups before the concave mirror provide enough power to image theentrance pupil at infinity at the aperture stop at or near the concavemirror. The third lens group after the concave mirror provides asubstantial portion of the reduction from object to image of the opticalsystem, as well as projecting the aperture stop to an infinite exitpupil. High-order aberrations are reduced by using an aspheric concavemirror.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention. In the drawings:

FIG. 1 is a schematic illustration of a conventional optical projectionsystem.

FIG. 2A is an illustration of diffraction at the reticle.

FIG. 2B is an illustration of the properties of a quarter-wave plate.

FIG. 2C is an illustration of the properties of a half waveplate.

FIG. 3 is a schematic illustration of the present invention using morethan two quarter-wave plates.

FIG. 4 is a schematic illustration of an alternative embodiment.

FIG. 5 is a schematic illustration of one embodiment of the presentinvention using a single refracting material.

FIG. 6 is another embodiment of the present invention using twodifferent refracting materials.

FIG. 7 is another embodiment of the present invention using more thantwo different refracting materials.

FIG. 8 is another embodiment of the present invention.

FIG. 9 is yet a further embodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION OF THE INVENTION

I. Overview

A. Conventional Optical System

B. Reticle Diffraction

C. Polarization and Wave Plates

II. Terminology

III. Example Implementations

A. Optical System With Elimination of Reticle Diffraction Induced Bias

B. Alternate Embodiment

C. Further Embodiments

IV. Alternate Implementation

I. Overview

A. Conventional Optical System

FIG. 1 illustrates a conventional optical reduction system. From itslong conjugate end where the reticle is placed to its short conjugateend where the wafer is placed, it possesses a first optical componentgroup 120, a beamsplitter cube 150, a first quarter-wave plate 140, aconcave mirror 130, a second quarter-wave plate 160, and a secondoptical component group 170. A feature of any optical system is theinterdependence of numerical aperture size and spectral radiationrequirements. In order to efficiently illuminate the image or waferplane 180, linearly polarized light is used. The limitations of linearlypolarized light are introduced above and discussed in the followingsections.

B. Reticle Diffraction Induced Bias

As recognized by the present inventor, the use of linearly polarizedlight at numerical apertures greater than about 0.5 introduces small,but noticeable, asymmetries in the imaging. These asymmetries in imagingare caused at least in part by diffraction of the linearly polarizedlight at certain feature orientations. FIG. 2A illustrates theasymmetries or print biases which result from the use of linearlypolarized light at the reticle 110. Simply, reticle 110 is placed in thepath of both linearly polarized light 205 and circularly polarized light210. The two types of light are separated by separator 215. After thereticle, the intensity of the light is distributed differently, as shownby distribution curves 220 and 225. The results are shown on wafer 180.Here, the projected image 230 resulting from the use of linearlypolarized light 205 is not as clear or sharp as projected image 235which results from the use of circularly polarized light 210.

Circularly polarized light 210 is indistinguishable from unpolarizedlight in its imaging behavior. The imaging behavior of unpolarized lightis such that it diffracts equally regardless of the orientation of thefeature through which it is projected. When the projection optic cannotaccept unpolarized light, but requires linearly polarized light, it ispossible to provide circularly polarized light to illuminate the reticleand thereby eliminate the feature orientation bias. Thus, print biasesare reduced.

C. Polarization and Wave Plates

The properties of wave plates are shown in FIGS. 2B and 2C. FIG. 2Billustrates the properties of a quarter-wave plate. Linearly polarizedinput 240 enters the wave plate 245 at the input polarization plane 255.The optic axis 250 and other factors discussed in detail below determinethe orientation of the output light. Here, wave plate 245 is designed toproduce circularly polarized output 260.

Similarly, FIG. 2C illustrates the properties of a half-wave plate.Linearly polarized input 265 enters the wave plate 270 at the inputpolarization plane 280. The optic axis 275 and other factors discussedin detail below determine the orientation of the output light. Here,wave plate 270 is designed to produce linearly polarized with plane ofpolarization retarded output 285.

Wave plates (retardation plates or phase shifters) are made frommaterials which exhibit birefringence. Birefringent materials, such ascrystals, are generally anisotropic. This means that the atomic bindingforces on the electron clouds are different in different directions andas a result so are the refractive indices.

In the case of uniaxial birefringent crystals, a single symmetry axis(actually a direction) known as the optic axis (shown in FIGS. 2B and 2Cas elements 250 and 275, respectively) displays two distinct principalindices of refraction: the maximum index n_(o) (the slow axis) and theminimum index n_(e) (the fast axis). These two indices correspond tolight field oscillations parallel and perpendicular to the optic axis.

The maximum index results in ordinary rays passing through the material.The minimum index results in extraordinary rays passing through thematerial. The velocities of the extraordinary and ordinary rays throughthe birefringent materials vary intensely with their refractive indices.The difference in velocities gives rise to a phase difference when thetwo beams recombine. In the case of an incident linearly polarized beamthis is given by:${\alpha = {2\quad\pi\quad d\frac{\left( {n_{e} - n_{o}} \right)}{\lambda}}};$where α is the phase difference; d is the thickness of wave plate;n_(e), n_(o) are the refractive indices of the extraordinary andordinary rays respectively, and λ is the wavelength. Thus, at anyspecific wavelength the phase difference is governed by the thickness ofthe wave plate.

As discussed above, FIG. 2B illustrates the operation of a quarter-waveplate. The thickness of the quarter-wave plate is such that the phasedifference is ¼-wavelength (zero order) or some multiple of ¼ wavelength(multiple order). If the angle between the electric field vector of theincident linearly polarized beam and the retarder principal plane of thequarter-wave plate is 45 degrees, the emergent beam is circularlypolarized.

Additionally, when a quarter-wave plate is double passed, e.g., when thelight passes through it twice because it is reflected off a mirror, itacts as a half-wave plate.

By quarter-wave plate is meant a thickness of birefringent materialwhich introduces a quarter of a wavelength of the incident light. Thisis in contrast to an integral number of half plus quarter-waves or twothicknesses of material whose phase retardance differs by aquarter-wave. The deleterious effects of large angle of incidencevariations are thereby minimized at the high numerical aperture by theuse of such zero order wave plates, and by restricting the field size inthe plane of incidence.

Similarly, FIG. 2C illustrates the operation of a half-wave plate. Thethickness of a half-wave plate is such that the phase difference is½-wavelength (zero order) or some odd multiple of ½-wavelength (multipleorder). A linearly polarized beam incident on a half-wave plate emergesas a linearly polarized beam but rotated such that its angle to theoptical axis is twice that of the incident beam.

II. Terminology

To more clearly delineate the present invention, an effort is madethroughout the specification to adhere to the following term definitionsas consistently as possible.

-   The term “circuitry” refers to the features designed for use in a    semiconductor device.-   The term “feature orientation” refers to the patterns printed on a    reticle to projection.-   The term “long conjugate end” refers to the plane at the object or    reticle end of the optical system.-   The term “print bias” refers to the variations in the lines on the    wafer produced by asymmetries in the optical system. Asymmetries are    produced by diffraction at various stages of the system and the    reticle.-   The term “semiconductor” refers to a solid state substance that can    be electrically altered.-   The term “semiconductor chip” refers to semiconductor device    possessing any number of transistors or other components.-   The term “semiconductor device” refers to electronic equipment    possessing semiconductor chips or other elements.-   The term “short conjugate end” refers to the plane at the image or    wafer end of the optical system.-   The term “wave plate” refers to retardation plates or phase shifters    made from materials which exhibit birefringence.    III. Example Implementations

A. Optical System With Elimination of Reticle Diffraction Induced Bias

The present invention uses circularly polarized light to eliminate thereticle diffraction induced biases of conventional systems. FIG. 3illustrates an embodiment of the present invention that eliminates suchasymmetries or print biases. A first quarter-wave plate 305 isintroduced before the object or reticle plane 110. First quarter-waveplate 305 converts the linearly polarized light into circularlypolarized light, as illustrated in FIG. 2B. As discussed above,circularly polarized light is indistinguishable from unpolarized lightin its imaging behavior. The imaging behavior of unpolarized light issuch that it diffracts equally regardless of the orientation of thefeature through which it is projected. Thus the print biases whichresult from reticle diffraction are reduced.

In order to minimize transmission loss through the rest of the opticalsystem, second quarter-wave plate 315 is inserted to linearly polarizethe radiation before the optical component group 320.

With respect to quarter-wave plates 305, 315, 340 and 360, oneorientation is to have the first quarter-wave plate 305 oriented withits fast axis parallel to that of the input light. The secondquarter-wave plate 315 and fourth quarter-wave plate 360 have their fastaxes in a parallel orientation but perpendicular to the fast axis ofthird quarter-wave plate 340.

B. Alternate Embodiment

It is also apparent to one skilled in the relevant art that secondquarter-wave plate 315 could be inserted into the system anywhere beforethe beamsplitter 350. This aspect is shown in FIG. 4 where secondquarter-wave plate 425 serves the same function. The transmission losscaused by the use of circularly polarized light within optical componentgroup 320 influences the placement of second quarter-wave plate 425.

Specifically, the use of unpolarized or circularly polarized light atthe beamsplitter would cause a transmission loss of 50%. If anon-polarized beamsplitter were to be used, 75% of the light would belost. Therefore, while alternate embodiments are possible, they may notbe feasibly implemented.

With respect to quarter-wave plates 405, 425, 440 and 460, oneorientation is to have the first quarter-wave plate 405 oriented withits fast axis parallel to that of the input light. The secondquarter-wave plate 425 and fourth quarter-wave plate 460 have their fastaxes in a parallel orientation but perpendicular to the fast axis ofthird quarter-wave plate 440.

C. Further Embodiments

The present invention can be implemented in various projection opticsystems. For example, the present invention can be implemented incatadioptric systems as described in detail herein, as well asrefractive and reflective systems. On skilled in the relevant art, basedat least on the teachings provided herein, would recognize that theembodiments of the present invention are applicable to other reductionsystems. More detailed embodiments of the present invention as providedbelow.

FIG. 5 illustrates one embodiment, of the optical reduction system ofthe present invention. From its long conjugate end, it comprises anfirst quarter-wave plate 508, an object or reticle plane 110, a secondquarter-wave plate 511, a first lens group LG1, a folding mirror 520, asecond lens group LG2, a beamsplitter cube 530, a third quarter-waveplate 532, a concave mirror 534, a second quarter-wave plate 538, and athird lens group LG3. The image is formed at image or wafer plane 180.The first lens group LG1 comprises a shell 512, a spaced doubletincluding positive lens 514 and negative lens 516, and positive lens518. The shell 512 is an almost zero power or zero power lens. Thesecond lens group LG2 comprises a positive lens 522, a spaced doubletincluding a negative lens 524 and a positive lens 526, and negative lens528. The third lens group LG3 comprises two positive lenses 540 and 542,which are strongly positive, shell 544, and two positive lenses 546 and548, which are weakly positive. The first quarter-wave plate 508 passescircularly polarized light incident upon the object or reticle plane110. The folding mirror 520 is not essential to the operation of thepresent invention. However, the folding mirror permits the object andimage planes to be parallel which is convenient for one intendedapplication of the optical system of the present invention, which is themanufacture of semiconductor devices using photolithography with a stepand scan system.

Radiation enters the system at the long conjugate end and passes throughthe first lens group LG1, is reflected by the folding mirror 520, andpasses through the second lens group LG2. The radiation enters thebeamsplitter cube 530 and is reflected from surface 536 passing throughquarter-wave plate 532 and reflected by concave mirror 534. Theradiation then passes back through the quarter-wave plate 532, thebeamsplitter cube 530, the quarter-wave plate 538, lens group LG3, andis focused at the image or wafer plane 180.

Lens groups upstream of the mirror, LG1 and LG2, provide only enoughpower to image the entrance pupil at infinity to the aperture stop 531at or near the concave mirror 534. The combined power of lens groups LG1and LG2 is slightly negative. The shell 512 and air spaced doublet 514and 516 assist in aberration corrections including astigmatism, fieldcurvature, and distortion. The lens group LG3, after the concave mirror534, provides most of the reduction from object to image size, as wellas projecting the aperture stop to an infinite exit pupil. The twostrongly positive lenses 540 and 542 provide a high numerical apertureat the image and exit pupils and infinity. The shell 544 has almost nopower. The two weakly positive lenses 546 and 548 help correct highorder aberrations. The concave mirror 534 may provide a reduction ratioof between 1.6 and 2.7 times that of the total system.

The negative lens 524 in the second lens group LG2 provides a stronglydiverging beam directed at the beamsplitter cube 530 and concave mirror534. The strongly positive lens 522 provides lateral color correction.The air space doublet comprising lenses 524 and 526 helps to correctspherical aberrations and coma. Concave mirror 534 is preferablyaspheric, therefore helping further reduce high order aberrations.

The transmission losses introduced by the beamsplitter cube 530 areminimized by illuminating the object or reticle with linearly polarizedlight and including a quarter-wave plate 532 between the beamsplittercube 530 and the concave mirror 534. Additionally, by increasing thenumerical aperture in lens group LG3, after the concave mirror 534 andbeamsplitter cube 530, the greatest angular range is not seen in theseelements.

However, the use of linearly polarized light at numerical aperturesgreater than about 0.5 introduces small but noticeable asymmetries inthe imaging. In the present invention, this can effectively be removedby introducing another quarter-wave plate 538 after the final passagethrough the beamsplitter cube 530, thereby converting the linearlypolarized light into circularly polarized light. This circularlypolarized light is basically indistinguishable from unpolarized light inits imaging behavior.

The optical system illustrated in FIG. 5 is designed to operate at areduction ratio of 4 to 1. Therefore, the numerical aperture in theimage space is reduced from 0.7 by a factor of 4 to 0.175 at the objector reticle plane 110. In other words, the object space numericalaperture is 0.175 and the image space numerical aperture is 0.7. Uponleaving the first lens group LG1 the numerical aperture is reduced to0.12, a consequence of the positive power needed in lens group LG1 toimage the entrance pupil at infinity to the aperture stop of the systemclose to the concave mirror 534. The numerical aperture after leavingthe second lens group LG2 and entering the beamsplitter is 0.19.Therefore, the emerging numerical aperture from the second lens groupLG2, which is 0.19, is larger than the entering or object spacenumerical aperture of lens group LG1, which is 0.175. In other words,the second lens group LG2 has an emerging numerical aperture greaterthan the entering numerical aperture of the first lens group LG1. Thisis very similar to the object space numerical aperture, which is 0.175,due to the overall negative power of the second lens group LG2. This iscontrary to prior art systems where the numerical aperture entering abeamsplitter cube is typically close to zero or almost collimated. Theconcave mirror 534 being almost concentric, the numerical aperture ofthe radiation reflected from it is increased only slightly from 0.19 to0.35. The third lens group LG3 effectively doubles the numericalaperture to its final value of 0.7 at the wafer or image plane 180.

The present invention achieves its relatively high numerical aperturewithout obstruction by the edges of the beamsplitter cube by means ofthe negative second group LG2 and the strongly positive third lens groupLG3. The use of the beamsplitter cube 530 rather than a platebeamsplitter is important in the present invention because at numericalapertures greater than about 0.45 a beamsplitter cube will providebetter performance. There is a reduction of the numerical aperturewithin the cube by the refractive index of the glass, as well as theabsence of aberrations that would be introduced by a tilted platebeamsplitter in the non-collimated beam entering the beamsplitter. Theconstruction data for the lens system illustrated in FIG. 5 according tothe present invention is given in Table 1 below. TABLE 1 Element Radiusof Curvature (mm) Thickness Aperture Diameter (mm) Number Front Back(mm) Front Back Glass 508 Infinite Infinite 4.500 123.0000 123.0000Silica Space 0.7500 110 Infinite 63.3853 Space 0.7500 511 InfiniteInfinite 4.500 123.0000 123.0000 Silica Space 0.7500 512 −158.7745−177.8880 15.0000 124.0478 131.7725 Silica Space 36.1130 514 −556.6911−202.0072 22.2126 148.3881 152.5669 Silica Space 38.7188 516 −183.7199−558.8803 15.0000 156.5546 166.5750 Silica Space 10.0674 518 427.2527−612.2450 28.8010 177.4010 179.0292 Silica Space 132.3320 520 Infinite−74.0000 184.6402 Reflection 522 −240.4810 2050.9592 −33.3135 188.4055185.3395 Silica Space −29.3434 524 421.7829 −145.6176 −12.0000 175.5823169.0234 Silica Space −4.2326 526 −150.4759 472.0653 −46.5091 171.4244169.9587 Silica Space −2.0000 528 −1472.2790 −138.2223 −15.0000 165.3586154.8084 Silica Space −27.2060 530 Infinite Infinite −91.8186 155.6662253.0917 Silica 536 Infinite 253.0917 Reflection 530 Infinite Infinite91.8186 253.0917 253.0917 Silica Space 2.0000 532 Infinite Infinite6.0000 185.8693 186.8401 Silica Space 17.9918 Stop 188.0655 534 Aspheric−17.9918 188.0655 Reflection 532 Infinite Infinite −6.0000 183.5471180.1419 Silica Space −2.0000 530 Infinite Infinite −91.8186 178.3346149.2832 Silica 530 Infinite Infinite −70.000 149.2832 128.8604 SilicaSpace −2.0000 538 Infinite Infinite −4.500 127.9681 126.6552 SilicaSpace −0.7500 540 −175.1330 1737.4442 −17.7754 121.4715 118.2689 SilicaSpace −0.7500 542 −108.8178 −580.1370 −18.2407 104.5228 97.7967 SilicaSpace −0.7500 544 −202.2637 −86.6025 −31.1216 91.7061 57.4968 SilicaSpace −2.3507 546 −122.1235 −488.7122 −17.9476 56.4818 41.1675 SilicaSpace −0.2000 548 −160.8506 −360.1907 −6.1500 39.4528 33.5764 SilicaSpace −4.000 180 Infinite  26.5019

Concave mirror 534 has an aspheric reflective surface according to thefollowing equation:${Z = {\frac{({CURV})Y^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)({CURV})^{2}Y^{2}}}} + {(A)Y^{4}} + {(B)Y^{6}} + {(D)Y^{10}} + {(E)Y^{12}} + {(F)Y^{14}}}};$wherein the constants are as follows:

CURV=−0.00289051

K=0.000000

A=6.08975×10⁻¹¹

B=2.64378×10¹⁴

C=9.82237×10⁻¹⁹

D=7.98056×10⁻²³

E=−5.96805×10⁻²⁷

F=4.85179×10⁻³¹

The lens according to the construction in Table 1 is optimized forradiation centered on 248.4 nanometers. The single refracting materialof fused silica and the large portion of refracting power restricts thespectral bandwidth of the embodiment illustrated in FIG. 5 to about 10picometers or 0.01 nanometers. This spectral bandwidth is more thanadequate for a line narrowed krypton fluoride excimer laser lightsource. The embodiment illustrated in FIG. 5 can be optimized for anywavelength for which fused silica transmits adequately.

A wider spectral bandwidth can be achieved by the use of two opticalmaterials with different dispersions. A second embodiment of the presentinvention is illustrated in FIG. 6. From its long conjugate end, itcomprises a first quarter-wave plate 608, an object or reticle plane110, a second quarter-wave plate 611, a lens group LG4, a folding mirror622, a lens group LG5, a beamsplitter cube 632 having surface 638, athird quarter-wave plate 634, a concave mirror 636, a fourthquarter-wave plate 640, and lens group LG6. The image is formed at imageor wafer plane 180. The lens group LG4 comprises a spaced doubletincluding negative lens 612 and positive lens 614, a weak positive lens616, positive lens 618, and shell 620. The lens group LG5 comprises apositive lens 624, a negative lens 626, a positive lens 628, and anegative lens 630. The lens group LG6 comprises two positive lenses 642,cemented doublet including positive lens 644 and negative lens 646,positive lens 648, and cemented doublet including shell 650 and positivelens 652.

This second embodiment uses calcium fluoride in one of the individualpositive lenses of the lens group LG4, negative lenses of the lens groupLG5, and two of the positive lenses of the lens group LG6. Theconstruction data of the second embodiment illustrated in FIG. 6 of thepresent invention is given in Table 2 below. TABLE 2 Element Radius ofCurvature (mm) Thickness Aperture Diameter (mm) Number Front Back (mm)Front Back Glass 608 Infinite Infinite 4.5000 123.0000 123.0000 SilicaSpace 0.5000 110 Infinite 60.4852 Space 0.5000 611 Infinite Infinite4.5000 123.0000 123.0000 Silica 612 −205.5158 539.1791 15.2158 124.0926137.3346 Silica Space 8.8054 614 2080.9700 −210.6539 32.4984 142.6149151.7878 Silica Space 1.2676 616 310.4463 700.3748 40.7304 162.4908165.2126 CaFl Space 0.5000 618 634.1820 −798.8523 27.5892 165.4595166.4747 Silica Space 0.5000 620 1480.0597 1312.1247 25.4322 168.7516164.7651 Silica Space 136.2343 622 Infinite −74.0000 161.9590 Reflection624 −761.9176 1088.9351 −19.2150 160.3165 159.2384 Silica Space −19.9465626 648.8361 −202.5872 −12.0000 155.1711 153.0635 CaFl Space −7.6304 628−400.4276 458.5060 −25.8769 153.0635 153.8055 Silica Space −2.0000 630−818.0922 −168.5034 −27.5927 152.6663 147.5200 CaFl Space −20.5014 632Infinite Infinite −91.7553 148.6158 252.7349 Silica 638 Infinite252.7349 Reflection 632 Infinite Infinite 91.7553 252.7349 252.7349Silica Space 2.0000 634 Infinite Infinite 6.0000 185.8070 187.0026Silica Space 18.1636 Stop 188.5681 636 Aspheric −18.1636 188.5681Reflection 634 Infinite Infinite −6.0000 184.2566 181.1084 Silica Space−2.0000 632 Infinite Infinite −91.7553 179.3838 151.7747 Silica 632Infinite Infinite −70.0000 151.7747 133.3985 Silica Space −2.0000 640Infinite Infinite −4.5000 132.5690 131.3876 Silica Space −0.5000 642−112.0665 −597.6805 −21.4866 123.4895 119.2442 Silica Space −0.5000 644−116.3137 282.3140 −24.0940 107.8451 101.2412 CaFl 646 282.3140 −66.5293−13.7306 101.2412 72.6862 Silica Space −2.6346 648 −77.2627 −374.4800−17.9594 72.0749 62.7659 Silica Space −0.5452 650 −130.1381 −57.1295−20.8147 58.9696 37.4889 Silica 652 −57.1295 −7305.8777 −6.1425 37.488934.3156 CaFl Space −4.0000 180 Infinite  26.4992wherein the constants for the aspheric mirror 634 used in the equationafter Table 1 are as follows:

CURV=−0.00286744

K=0.000000

A=−1.92013×10⁻⁰⁹

B=−3.50840×10⁻¹⁴

C=2.95934×10⁻¹⁹

D=−1.10495×10⁻²²

E=9.03439×10⁻²⁷

F=−1.39494×10⁻³¹

This second embodiment is optimized for radiation centered on 193.3nanometers and has a spectral bandwidth of about 200 picometers or 0.2nanometers. A slightly line narrowed argon fluoride excimer laser is anadequate light source. Additionally, the design can be optimized for anywavelength for which both refractive materials transmit adequately. Thebandwidth will generally increase for longer wavelengths, as thematerial dispersions decrease. For example, around 248.4 nanometers sucha two-material design will operate over at least a 400 picometers, 0.4nanometers bandwidth.

At wavelengths longer than 360 nanometers, a wider range of opticalglasses begin to have adequate transmission. A third embodimentillustrated in FIG. 7 takes advantage of this wider selection of glassesand further reduced dispersion. From its long conjugate end, itcomprises a first quarter-wave plate 708, an object or reticle plane110, a second quarter-wave plate 711, a lens group LG7, a folding mirror722, a lens group LG8, a beamsplitter cube 732 having a surface 738, athird quarter-wave plate 734, a concave mirror 736, a fourthquarter-wave plate 740, and lens group LG9. The image is formed at imageor wafer plane 180. The lens group LG7 comprises a spaced doubletcomprising negative lens 712 and positive lens 714, spaced doubletincluding positive lens 716 and negative lens 718, and positive lens720. The lens group LG8 comprises a positive lens 724, a negative lens726, a positive lens 728, and a negative lens 730. The lens group LG9comprises a positive lenses 742, cemented doublet including positivelens 744 and negative lens 746, positive lens 748, and cemented doubletincluding shell 750 and positive lens 752.

The construction data of the third embodiment illustrated in FIG. 7 isgiven in Table 3 below. TABLE 3 Element Radius of Curvature (mm)Thickness Aperture Diameter (mm) Number Front Back (mm) Front Back Glass708 Infinite Infinite 4.5000 125.0000 125.0000 Silica Space 0.5000 110Infinite 59.2960 Space 0.5000 711 Infinite Infinite 4.5000 125.0000125.0000 Silica 712 −620.7809 361.8305 20.2974 125.9406 134.7227 PBM2YSpace 2.6174 714 515.7935 −455.1015 39.8858 135.3384 145.6015 PBM2YSpace 14.7197 716 431.3189 −239.4002 36.9329 155.6269 157.3014 BSL7YSpace 0.5000 718 −259.6013 685.3286 26.3534 156.9363 162.2451 PBM2YSpace 1.4303 720 361.5709 −1853.2955 23.3934 168.7516 165.1801 BAL15YSpace 131.8538 722 Infinite −77.8469 169.9390 Reflection 724 −429.2950455.4247 −32.3086 173.0235 171.1102 PBL6Y Space −27.6206 726 401.0363−180.0031 −12.0000 159.3555 154.7155 BSL7Y Space −5.6227 728 −258.47221301.3764 −26.1321 154.7155 154.1517 PBM8Y Space −2.0000 730 −1282.8931−180.2226 −12.0000 153.1461 149.4794 BSL7Y Space −19.7282 732 InfiniteInfinite −91.7349 150.4585 252.6772 Silica 738 Infinite 252.6772Reflection 732 Infinite Infinite 91.7349 252.6772 252.6772 Silica Space2.0000 734 Infinite Infinite 6.0000 185.6435 186.7758 Silica Space18.2715 Stop 188.1745 736 Aspheric −18.2715 188.1745 Reflection 734Infinite Infinite −6.0000 183.6393 180.1377 Silica Space −2.0000 732Infinite Infinite −91.7349 178.3236 147.9888 Silica 732 InfiniteInfinite −70.0000 147.9888 126.9282 Silica Space −2.000 740 InfiniteInfinite −4.5000 126.0289 124.6750 Silica Space −0.5000 742 −119.8912−610.6840 −18.6508 117.5305 113.4233 BSM51Y Space −0.5000 744 −114.1327384.9135 −21.1139 102.6172 96.4137 BSL7Y 746 384.9135 −70.2077 −13.057696.4137 71.1691 PBL26Y Space −2.8552 748 −85.7858 −400.3240 −16.914770.5182 61.2633 BSM51Y Space −0.8180 750 −151.5235 −54.0114 −19.581057.6234 37.3909 BSM51Y 752 −54.0114 −2011.1057 −6.3947 37.3909 34.2119PBL6Y Space −4.0000 180 Infinite  26.5002wherein the constants for the aspheric mirror 736 used in the equationafter Table 1 as follows:

CURV=−0.00291648

K=0.000000

A=−1.27285×10⁻⁹

B=−1.92865×10⁻¹⁴

C=6.21813×10⁻¹⁹

D=−6.80975×10²³

E=6.04233×10⁻²⁷

F=3.64479×10⁻³²

This third embodiment operates over a spectral bandwidth of 8 nanometerscentered on 365.5 nanometers. A radiation of this spectral bandwidth canbe provided by a filtered mercury arc lamp at the I-line waveband. Theoptical glasses other than fused silica used in this third embodimentare commonly known as I-line glasses. These optical glasses have theleast absorption or solarization effects at the mercury I-linewavelength.

FIG. 8 illustrates a fourth embodiment of the optical reduction systemof the present invention. This embodiment has a numerical aperture of0.63 and can operate at a spectral bandwidth of 300 picometers, andpreferably of 100 picometers, centered on 248.4 nanometers. From thelong conjugate end, it includes a first quarter-wave plate 808, anobject or reticle plane 110, a second quarter-wave plate 811, a firstlens group LG1, a folding mirror 820, a second lens group LG2, abeamsplitter cube 830, a first quarter-wave plate 832, a concave mirror834, a second quarter-wave plate 838, and a third lens group LG3. Theimage is formed at the image or wafer plane 180.

The first lens group LG1 comprises a shell 812, a spaced doubletincluding a positive lens 814 and a negative lens 816, and a positivelens 818. The second lens group LG2 comprises a positive lens 822, aspaced doublet including a negative lens 824 and a positive lens 826,and a negative lens 828. The third lens group LG3 comprises two positivelenses 840 and 842, a shell 844, and two positive lenses 846 and 848.Again, as in the embodiment illustrated in FIG. 5, the folding mirror820 of FIG. 8 is not essential to the operation of the invention, butnevertheless permits the object 110 and image plane 180 to be parallelto each other which is convenient for the manufacture of semiconductordevices using photolithography.

The construction data of the fourth embodiment illustrated in FIG. 8 isgiven in Table 4 below. TABLE 4 Element Radius of Curvature (mm)Thickness Aperture Diameter (mm) Number Front Back (mm) Front Back Glass808 Infinite Infinite 4.5000 122.0000 122.0000 Silica Space 2.0000 110Infinite 63.3853 Space 2.0000 811 Infinite Infinite 4.5000 122.0000122.0000 Silica 812 −183.5661  −215.7867CX 17.0000 122.8436 130.6579Silica Space 46.6205 814 −601.1535CC  −230.9702CX 21.4839 149.1476153.3103 Silica Space 68.8075 816 −195.1255  −345.4510CX 15.0000161.6789 170.1025 Silica Space 3.0000 818  435.8058CX −1045.1785CX24.9351 177.4250 178.2672 Silica Space 130.0000 Decenter(1) 820 Infinite−64.5000 180.3457 Reflection 822 −210.7910CX  380.1625CX −43.1418181.6672 178.0170 Silica Space −15.8065 824  300.1724CC  −123.4555CC−12.0000 166.7278 152.3101 Silica Space −3.8871 826 −126.8915CX 972.6391CX −41.3263 154.8530 151.8327 Silica Space −1.5000 828−626.4905CX  −116.6456CC −12.0000 147.6711 136.1163 Silica Space−31.8384 830 Infinite Infinite −74.0000 137.2448 200.1127 SilicaDecenter(2)| 836 Infinite 200.1128 Reflection 830 Infinite Infinite74.0000 200.1127 200.1127 Silica Space 2.0000 832 Infinite Infinite6.0000 148.6188 149.0707 Silica Space 14.4638 Stop 149.6392 834 Aspheric−14.4638 149.6392 Reflection 832 Infinite Infinite −6.0000 144.8563141.2737 Silica Space −2.0000 830 Infinite Infinite −74.000 139.3606117.3979 Silica Decenter(3) 830 Infinite Infinite −61.000 117.3979100.5074 Silica Space −2.0000 838 Infinite Infinite −4.5000 99.661798.4157 Silica Space −1.2000 840 −157.8776CX  2282.2178CX −13.750194.8267 91.8775 Silica Space −1.2000 842  −94.0059CX  −46.6659CC−13.4850 82.8663 78.1418 Silica Space −1.2000 844 −147.2485CX −77.8924CC −22.2075 72.7262 50.6555 Silica Space −3.2091 846−159.2880CX  −519.4850CC −13.8321 49.5648 39.0473 Silica Space −0.2000848 −129.3683CX  −426.7350CC −6.1500 37.3816 32.4880 Silica Space ImageDistance = −4.0000 850 Image Infinite

The constants for the aspheric mirror 834 used in the equation locatedafter Table 1 are as follows:

CURV=−0.00332614

K=0.000000

A=−4.32261E−10

B=3.50228E−14

C=7.13264E−19

D=2.73587E−22

This fourth embodiment is optimized for radiation centered on 248.4 nm.The single refracting material of fused silica and the large portion ofrefracting power restricts the spectral bandwidth of the embodimentdepicted in FIG. 8. However, because the fourth embodiment has a maximumnumerical aperture of 0.63 rather than of 0.7 as in the first threeembodiments, the fourth embodiment provides acceptable imaging over aspectral full-width-half-maximum bandwidth of 300 picometers, orpreferably of 100 picometers. Thus, in the former, an unnarrowed, or, inthe latter, a narrowed excimer laser can be employed for theillumination source.

The fourth embodiment differs from the first three embodiments in thatthe net power of LG1 and LG2 of the fourth embodiment is weakly positiverather than weakly negative as in the first three embodiments. Inaddition, this illustrates that the overall focal power of LG1 plus LG2can be either positive or negative and still permit an infinitelydistant entrance pupil to be imaged at or near the concave mirror 834.

FIG. 9 illustrates a fifth embodiment of the optical reduction system ofthe present invention. Preferably, this embodiment has a numericalaperture of 0.60 and operates at a spectral bandwidth of 300 picometerscentered on 248.4 nanometers. From the long conjugate end, it includes afirst quarter-wave plate 908, an object or reticle plane 110, a secondquarter-wave plate 911, a first lens group LG1, a folding mirror 920, asecond lens group LG2, a beamsplitter cube 930, a third quarter-waveplate 932, a concave mirror 934, a fourth quarter-wave plate 938, and athird lens group LG3. The image is formed at an image or wafer plane180.

The first lens group LG1 comprises a shell 912, a spaced doubletincluding a positive lens 914 and a negative lens 916, and a positivelens 918. The second lens group LG2 comprises a positive lens 922, aspaced doublet including a negative lens 924 and a positive lens 926,and a negative lens 928. The third lens group LG3 comprises two positivelenses 940 and 942, a shell 944, and two positive lenses 946 and 948.Again, as in the embodiment illustrated in FIG. 5, the folding mirror920 of FIG. 9 is not essential to the operation of the invention, butnevertheless permits the object and image planes to be parallel to eachother which is convenient for the manufacture of semiconductor devicesusing photolithography.

The construction data of the fifth embodiment illustrated in FIG. 9 isgiven in Table 5 below. TABLE 5 Element Radius of Curvature (mm)Thickness Aperture Diameter (mm) Number Front Back (mm) Front Back Glass908 Infinite Infinite −4.4550 120.0000 120.0000 Silica Space 1.1880 910Infinite 62.7514 Space 1.1880 911 Infinite Infinite −4.4550 120.0000120.0000 Silica 912 −136.1154 CC  −152.5295 CX 16.8300 120.7552 129.4354Silica Space 4.5206 914 −270.1396CC  −191.8742 CX 20.5341 132.9152139.0377 Silica Space 90.8476 916 −188.9000CC  −284.7476CX 17.5000156.1938 165.6567 Silica Space 2.9700 918  433.8174 CX  −841.5599CX25.8293 173.8279 174.8334 Silica Space 149.4549 Decenter(1) 920 Infinite−61.0000 177.2183 Reflection 922 −190.3251CX −8413.4836CC −34.4584178.5071 174.2260 Silica Space −51.5487 924  690.5706CC  −146.4997CC−11.8800 150.4109 141.8021 Silica Space −10.6267 526 −265.9886CX 1773.5314CX −24.1851 142.1851 141.2400 Silica Space −1.5000 928−244.9899CX  −142.8558CC −11.8800 139.3290 133.8967 Silica Space−21.6411 930 Infinite Infinite −71.2800 134.3115 189.7826 SilicaDecenter(2) 936 Infinite 189.7826 Reflection 930 Infinite Infinite71.2800 189.7826 189.7826 Silica Space 1.9800 932 Infinite Infinite5.9400 142.3429 142.6707 Silica Space 18.5263 Stop 143.5034 934 Aspheric−18.5263 143.5034 Reflection 932 Infinite Infinite −5.9400 134.2788130.9398 Silica Space −1.9800 930 Infinite Infinite −71.2800 130.1221111.7247 Silica Decenter (3) 930 Infinite Infinite −60.4000 111.724796.1353 Silica Space −1.9800 938 Infinite Infinite −4.4550 95.356294.2064 Silica Space −1.1880 940 −127.4561CX −1398.8019CC −13.010490.4737 87.7002 Silica Space −1.1880 942  −98.8795CX  −424.1302CC−12.2874 80.7016 76.3270 Silica Space −1.1880 944 −132.0104CX −70.9574CC −17.8706 71.0789 53.4306 Silica Space −3.1246 946−123.1071CX  −585.4471CC −19.9496 52.6417 38.2256 Silica Space −0.1980948 −137.8349CX  −292.6179CX −6.0885 36.7251 31.8484 Silica Space ImageDistance = −4.0000 950 Image Infinite  26.5000

The constants for the aspheric mirror 934 used in the equation locatedafter Table 1 are as follows:

CURV=−0.00325995

K=0.000000

A=−6.91799E−10

B=5.26952E−15

C=6.10046E−19

D=1.59429E−22

This fifth embodiment is optimized for radiation centered on 248.4 nm.The single refracting material of fused silica and the large portion ofrefracting power restricts the spectral bandwidth of the embodimentdepicted in FIG. 9. However, because the fifth embodiment has a maximumnumerical aperture of 0.6 rather than of 0.7 as in the first threeembodiments, the fifth embodiment provides acceptable imaging over aspectral full-width-half-maximum bandwidth of 300 picometers. Thus, anunnarrowed excimer laser can be employed for an illumination source. Thefifth embodiment differs from the first three embodiments in that thenet power of LG1 and LG2 of the fifth embodiment is weakly positiverather than weakly negative as in the first three embodiments. Inaddition, this illustrates that the overall focal power of LG1 plus LG2can be either positive or negative and still permit an infinitelydistant entrance pupil to be imaged at or near the concave mirror 934.

IV. Alternate Implementation

It is apparent to one skilled in the relevant art that the use of thefirst quarter-wave plate in any of the above embodiments depends on theinitial polarization of the radiation incident on the long conjugateend. Therefore, if the polarization of the light is circular orunpolarized prior to the long conjugate end, then the first quarter-waveplate, used to transform linearly polarized light into circularlypolarized light, could be omitted.

Such an implementation can be shown by omitting first quarter-wave plate305 from FIG. 3 and/or first quarter-wave plate 405 from FIG. 4. Furtherimplementations of this configuration in the other embodiments describedabove are obvious to one skilled in the relevant art.

Conclusion

While specific embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedin the appended claims. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A method for producing a lithography beam, comprising: generating anexposure beam; polarizing the exposure beam to produce a circularlypolarized exposure beam; and illuminating a reticle with the circularlypolarized exposure beam to produce an output beam having feature detailsof the reticle.
 2. The method of claim 1, further comprising: exposing awafer with the output beam.
 3. The method of claim 1, wherein thepolarizing step includes passing the exposure beam through aquarter-wave plate.
 4. The method of claim 1, further comprising:polarizing the output beam to produce a linearly polarized output beam.5. The method of claim 4, wherein the second polarizing step includespassing the exposure beam through a quarter-wave plate.
 6. The method ofclaim 4, further comprising: exposing a wafer with the linearlypolarized output beam.
 7. The method of claim 1, further comprising:correcting aberrations in at least one aberration selected from thegroup of: astigmatism, field curvature, distortion, sphericalaberration, and coma.